Colloidal ternary group III-V nanocrystals synthesized in molten salts

ABSTRACT

Methods of synthesizing colloidal ternary Group III-V nanocrystals are provided. Also provided are the colloidal ternary Group III-V nanocrystals made using the methods. In the methods, molten inorganic salts are used as high temperature solvents to carry out cation exchange reactions that convert binary nanocrystals into ternary nanocrystals.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. provisional patentapplication No. 62/690,035 that was filed Jun. 26, 2018, the entirecontents of which are incorporated herein by reference.

REFERENCE TO GOVERNMENT RIGHTS

This invention was made with government support under DMR-1611371awarded by the National Science Foundation. The government has certainrights in the invention.

BACKGROUND

Group III-V compounds, including GaAs, GaN, InGaP, and InGaAs, arearguably one of the most important classes of semiconductors, along withSi, due to their direct band gaps and superior electronic properties.One of the salient features of these compounds is the ability to makealloys with desired compositions that not only allow the band gaps to beprecisely tuned but also allow epitaxial growth of sandwich typearchitectures essential for device function. For instance, epitaxiallymatched InGaP/GaAs stacks are well known active layers for highefficiency solar cells. Similarly, InGaAs can be grown on an epitaxiallymatched InP substrate for infrared detector applications.

Solution processed colloidal semiconductor nanoparticles, also known ascolloidal quantum dots (QDs), have attracted a lot of attention asbuilding blocks for bottom-up assembly of thin film devices. ColloidalQDs of Group III-V compounds have been shown to be promising candidatesfor a plethora of applications such as display technology,light-emitting diodes (LEDs), photovoltaics, photodetectors, andbioimaging. In recent years, InP has replaced CdSe as the material ofchoice in QD-based displays due to its lower toxicity and comparableoptical properties. Simultaneously, other members of the Group III-Vfamily, such as InAs and InSb, have also seen a significant surge ininterest as infrared active materials for applications like bioimaging,night vision, and telecommunication.

Even though significant advances have been made in the syntheticchemistry of colloidal Group III-V semiconductors, this technology stilllags behind Group II-VI compounds. For example, whereas monodispersenanocrystals of InP and InAs can now be achieved, colloidal GaAsnanocrystals are still difficult to synthesize and show crystallinedefects when synthesized at temperatures relevant for colloidalchemistry. The more covalent character of Ga pnictides and the highoxophilicity of Ga make the synthesis of Ga containing Group III-Vcompounds challenging.

Ternary Group III-V nanocrystals (NCs), e.g., InGaP and InGaAs, aretechnologically interesting compounds due to the flexibility they offerin terms of band gap engineering. For example, an ensemble of InGaPnanoparticles emitting green light will be more efficient and stablethan their InP counterparts due to their larger size, and hence largerabsorption coefficients, which is directly proportional to the number ofunit cells in the nanocrystal. Moreover, incorporation of Ga in the InPlattice reduces its lattice mismatch with wider gap shell materials suchas ZnS, making the core-shell material less strained. Materials with agraded alloy composition are potentially interesting for applicationswith enhanced absorption coefficients in the blue (450 nm). From thestand point of emission, core-shell nanocrystals with a GroupIII-V/III-V interface are potentially better than Group III-V/II-VI coreshells for optical properties due to less interfacial strain. Therefore,for the efficient incorporation of Group III-V nanocrystals intocommercial technologies, the ability to engineer their composition willbe essential.

Group III-V nanocrystals with ternary alloy compositions such as InGaPor InGaAs have rarely been reported. (See, Micic, et al., The Journal ofPhysical Chemistry 1995, 99 (19), 7754-7759; Park, et al., Journal ofthe American Chemical Society 2016, 138 (51), 16568-16571; and Gerbec,et al., Journal of the American Chemical Society 2005, 127 (45),15791-15800. Attempts to make alloyed compositions have seen limitedsuccess. It has been shown that alloy compositions can only be achievedat a temperature of about ˜400° C. (Micic et al., 1995.) Efforts toalloy Ga into InP have resulted only in the surface exchange, and noalloying was observed. (Pietra et al., Chemistry of Materials 2017, 29(12), 5192-5199 and Kim et al., Journal of the American Chemical Society2012, 134 (8), 3804-3809. Regarding the thermodynamics of the exchange,such temperatures are typically difficult to reach with traditionalorganic solvents, which either boil or decompose at temperatures above350° C. It is also difficult to avoid side reactions such as oxidationof Ga compounds at such high temperatures.

SUMMARY

Methods of synthesizing colloidal ternary Group III-V nanocrystals areprovided. Also provided are the colloidal ternary Group III-Vnanocrystals made using the methods.

One embodiment of a method for forming ternary Group III-V nanocrystalsincludes the steps of: dispersing binary Group III-V nanocrystals in amolten inorganic salt; adding an ion-exchange additive comprising aGroup III element or a group V element to the molten inorganic salt; andheating the molten inorganic salt for a time and at a temperature atwhich the Group III element or the Group V element of the binary GroupIII-V nanocrystals and the Group III element or the Group V element ofthe ion-exchange additive undergo cation exchange to form the ternaryGroup III-V nanocrystals.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will hereafter be describedwith reference to the accompanying drawings.

FIG. 1A depicts a schematic showing a cation exchange process in moltensalts. The templating of molten salt ions around the QD surface isresponsible for stabilization of QDs in molten salt. Addition of GaI₃salt to the molten salt dispersion of InP or InAs QDs leads to cationexchange to form In_(1-x)Ga_(x)P or In_(1-x)Ga_(x)As QDs. FIG. 1Bdepicts the lattice constants and bulk band gaps (at 0 K) of alloys ofInP and GaP, InAs and GaAs, and ZnS and ZnSe. ZnS and ZnSe are typicallyused as the wide gap shell materials for InP QDs.

FIGS. 2A and 2B depict X-ray diffraction (XRD) patterns ofIn_(1-x)Ga_(x)P and In_(1-x)Ga_(x)As alloy QDs (also referred to asnanocrystals) obtained by cation exchange at different temperatures. Thevertical lines show the positions and intensities of X-ray reflectionsof bulk InP, GaP, InAs, and GaAs. FIGS. 2C and 2D depict Raman spectraof In_(1-x)Ga_(x)P and In_(1-x)Ga_(x)As alloy QDs, respectively,obtained by cation exchange at different temperatures. The verticallines show the corresponding TO and LO phonon modes of bulk InP, GaP,InAs, and GaAs.

FIG. 3A depicts a scanning transmission electron microscope (STEM) imageof In_(1-x)Ga_(x)P QDs. The inset shows a high-resolution STEM imagewith clearly visible lattice fringes. FIG. 3B depicts an energydispersive X-ray map and the corresponding STEM image showing ahomogenous distribution of indium and gallium in every particle. For Gaand P, the Kα edge was measured whereas for In, the Lα edge wasmeasured. FIGS. 3C and 3D depict TEM images of InP QDs and theIn_(0.6)Ga_(0.4)P alloy QDs. FIG. 3E depicts experimental small-angleX-ray scattering (SAXS) curves (open squares) and the fits (black lines)for InP and In_(0.6)Ga_(0.4)P QDs. The inset shows the sizedistributions extracted from the fits.

FIG. 4A depicts absorption spectra of starting InP QDs andIn_(1-x)Ga_(x)P QDs synthesized from InP QDs at different temperatures.FIG. 4B depicts absorption spectra of different sizes of InP QDs (dottedlines) and In_(1-x)Ga_(x)P QDs (solid lines) obtained from the InP QDsafter a cation exchange reaction at 400° C. FIG. 4C depicts absorptionand emission spectra of representative size-selected In_(1-x)Ga_(x)P QDsshowing stokes shifts of the emission bands. The full-width half maximumof the emission bands was 48 nm (green), 50 nm (orange) and 51 nm (red).FIG. 4D depicts a comparison of molar extinction coefficients (perparticle) of InP and In_(0.6)Ga_(0.4)P QDs with similar emissionprofiles (see inset).

FIG. 5A depicts absorption and photoluminescence (PL) spectra ofIn_(0.5)Ga_(0.5)P QD core (solid and dashed) and In_(0.5)Ga_(0.5)P/ZnScore-shell QDs (solid and dashed). The PL spectra depict the relativechange in PL intensity upon shell growth. FIG. 5B depicts representativePL spectra of In_(1-x)Ga_(x)P/ZnS core-shell QDs extending the rangebetween ˜490 nm and 640 nm, showing the range of emission wavelengthsaccessible by this material. FIG. 5C depicts a comparison ofphotoluminescence excitation (PLE) spectra for InP/ZnS QDs andIn_(0.5)Ga_(0.5)P/ZnS QDs. The corresponding PL spectra are shown indashed lines. PLE was measured at the corresponding emission maxima witha slit width of 2 nm. FIG. 5D depicts an XRD pattern ofIn_(0.6)Ga_(0.4)P/ZnS QDs. HR-TEM images of individualIn_(1-x)Ga_(x)P/ZnS QDs showed clear lattice fringes for the core-shellparticles.

FIG. 6A depicts temperature dependent PL spectra ofIn_(0.6)Ga_(0.4)P/ZnS QDs immobilized in a matrix of cross-linked poly(lauryl methacrylate). FIG. 6B depicts temperature dependent PL spectraof InP/ZnS QDs immobilized in a matrix of cross-linked poly (laurylmethacrylate). FIG. 6C shows the change in integrated PL area withincrease in temperature for In_(0.6)Ga_(0.4)P/ZnS QDs and InP/ZnS QDsimmobilized in a polymer matrix. FIG. 6D shows the change in integratedPL area with increase in temperature for In_(0.6)Ga_(0.4)P/ZnS QDs andInP/ZnS QD solutions.

FIG. 7A depicts absorption spectra of starting InAs QDs andIn_(1-x)Ga_(x)As QDs synthesized from InAs QDs at differenttemperatures. FIG. 7B depicts absorption spectra (solid lines) ofIn_(0.5)Ga_(0.5)As and In_(0.5)Ga_(0.5)As/CdS core-shell QDs and the PLspectrum (dashed) of In_(0.5)Ga_(0.5)As/CdS core-shell QDs.

FIGS. 8A and 8B depict TEM images of (FIG. 8A) starting InAs and (FIG.8B) resultant In_(0.5)Ga_(0.5)As QDs obtained by cation exchangeperformed at 450° C.

FIG. 9A depicts an HR-TEM image of In_(1-x)Ga_(x)P alloy QDs. FIG. 9Bdepicts an HR-TEM image of In_(1-x)Ga_(x)As alloy QDs.

FIG. 10 depicts L excitation spectra for In_(0.5)Ga_(0.5)P/ZnS QDsmonitored at different emission wavelengths.

FIGS. 11A and 11B depict temperature dependent PL spectra ofIn_(0.6)Ga_(0.4)P/ZnS QDs and InP/ZnS QDs dispersed in decane.

FIG. 12 depicts representative PL spectra of In_(1-x)Ga_(x)As/CdS QDs.

DETAILED DESCRIPTION

Methods of synthesizing colloidal ternary Group III-V nanocrystals areprovided. Also provided are the colloidal ternary Group III-Vnanocrystals made using the methods. In the methods, molten inorganicsalts are used as high temperature solvents to carry out cation exchangereactions that could not be carried out using traditional colloidalsolvents. This approach enables the synthesis of Group III-Vnanocrystals that would otherwise be difficult or impossible to achievein a colloidal nanocrystalline form. The ternary Group III-Vnanocrystals are quantum confined and, as such, have discrete opticalenergy spectra that are tunable over a wide range of wavelengths.

One embodiment of a method of making Group III-V nanocrystals includesthe steps of dispersing binary Group III-V nanocrystals in a molteninorganic salt; adding a salt of a Group III element or a salt of aGroup V element to the molten inorganic salt; and heating the molteninorganic salt, whereby the Group III or Group V element of the addedsalt undergoes cation exchange with the Group III or Group V element ofthe binary Group III-V nanocrystals, converting the nanocrystals intoternary Group III-V nanocrystals. (Alternatively, in the case of theGroup V elements, a gaseous Group V element-containing compound, such asNH₃ or PH₃, can be used as an exchange agent, rather than a salt.) Usingthis method, binary Group III-V nanocrystals can be transformed intoternary Group III-V nanocrystals without a significant change in theirsizes or morphologies. The ternary Group III-V nanocrystals have opticalproperties that render them well suited for use in a variety ofoptoelectronic applications, including display devices. These opticalproperties can be tuned by adjusting the particle size of the startingbinary nanocrystals and/or by tailoring the extent of the cationexchange and, therefore, the precise composition of the ternary alloys,as illustrated in the Example.

As used herein, the term “nanocrystal” or “nanoparticle” refers to aninorganic particle having a largest cross-sectional dimension of nogreater than 1000 nm. This includes particles having a largestcross-sectional dimension of no greater than 100 nm and further includesparticles having a largest cross-sectional dimension of no greater than10 nm. In some embodiments, the largest cross-sectional dimension is inthe range of about 1 nm to about 10 nm. When the nanocrystals arepresent as a plurality of nanocrystals, these dimensions refer to theaverage largest cross-sectional dimension for the collection ofnanocrystals. The nanocrystals can also be referred to as QDs.

A variety of molten inorganic salts may be used to form the colloidalternary Group III-V nanocrystals. The molten inorganic salt may be amixture (e.g., a eutectic mixture) of two or more different inorganicsalts. The selected molten inorganic salt should be one in which thebinary Group III-V nanocrystals can be dispersed and that has a highsolubility for the salt of the Group III or Group V element. Inaddition, the molten salt is desirably stable at high temperatures(e.g., temperatures >350° C.), has a low vapor pressure, and is inert to(i.e., does not interfere with) the cation exchange reaction between thebinary nanocrystals and the salt of the Group III or Group V element. Inaddition, the molten inorganic salt may be one that exhibits sufficientbinding affinity for the binary and ternary Group III-V nanocrystals soas to form a homogeneous, uniform dispersion of the nanocrystalsthroughout the molten inorganic salt. At least in some cases, thisbinding affinity may be sufficient to achieve the homogeneous, uniformdispersion even in the absence of any organic capping ligands associatedwith the nanocrystals. The phrase “binding affinity” can refer to theformation of covalent bonds or non-covalent (e.g., hydrogen bonds)between the molten inorganic salt (or a component thereof) and thenanocrystals.

In some embodiments, the molten inorganic salt is characterized by amelting point (T_(m)) of below 350° C. This includes embodiments inwhich the molten inorganic salt has a T_(m) in the range of from about50° C. to less than 350° C.

Suitable molten inorganic salts include metal halides, such as mixedmetal halides, and thiocyanate salts. Specific examples of metal halidesinclude CsBr—LiBr—KBr mixtures; LiCl—LiBr—KBr mixtures; LiCl—LiI—KImixtures; and ZnCl₂—NaCl—KCl mixtures. NaSCN—KSCN mixtures can also beused.

The colloids may be characterized as having a homogeneous and uniformdispersion of the ternary Group III-V nanocrystals throughout thecontinuous phase (i.e., the molten salt). Although the binary GroupIII-V nanocrystals initially may have organic capping ligands bound totheir surfaces, these organic capping ligands may be displaced withcomponents (e.g., ions) from the molten media while still providing astable dispersion. Generally, if the molten media has sufficientlystrong binding affinities for the binary Group III-V nanocrystals, theorganic capping ligands on the nanocrystals can be partially,completely, or substantially completely displaced by the ion of themolten media. In other embodiments, the colloids are formed using barebinary Group III-V nanocrystals, e.g., binary Group III-V nanocrystalsfrom which the organic ligands have been stripped. Optionally, theorganic ligands can be partially or completely replaced by inorganicligands, such as sulfide ligands (S²⁻ anions), that enhance thesolubility of the binary Group III-V nanocrystals in the molten salt.

Due to the replacement of organic capping ligands or the use of barebinary Group III-V nanocrystals, the colloids can be substantially freeof organic capping ligands. By “substantially free” it is meant that thecolloid is completely free of such ligands or such ligands are presentin such a small amount so as to have no material effect on the colloid.Similarly, in some embodiments, the colloids may be characterized asbeing substantially free of any capping ligands (e.g., inorganic cappingligands) other than those provided by the components of the moltenmedium itself.

Colloidal dispersions of the starting binary Group III-V nanocrystalscan be made by interfacing the molten inorganic salt with the binaryGroup III-V nanocrystals dispersed in an organic solvent and stirring,whereby the binary Group III-V nanocrystals undergo phase transfer fromthe organic solvent to the molten inorganic salt. Alternatively, thecolloids can be made by mixing the binary Group III-V nanocrystals in anon-polar solvent with the molten inorganic salt or through a“solvent-free” method. Details of these methods are described in theExample below and in PCT application publication number WO/2017105662,which is incorporated herein for the purpose of describing methods offorming colloids of inorganic nanocrystals in molten media.

Once a colloidal dispersion of the Group III-V nanocrystals has beenformed, an ion exchange additive that includes a Group III element or aGroup V element can be added to the dispersion at a temperature and fora time sufficient to exchange ions of the ion exchange additive withions of the binary Group III-V nanocrystals, thereby converting thebinary Group III-V nanocrystals into ternary Group III-V nanocrystals.The ion exchange additive may be a salt, or other compound, of a GroupIII element or a Group V element, wherein the Group III element or GroupV element of the salt differs from the Group III element or Group Velement of the binary nanocrystals. In some embodiments, the salt is ahalide salt of a Group III or Group V element. As illustrated in theExample, the chemical content of the ternary Group III-V nanocrystalscan be controlled by tailoring the degree of cation exchange throughcontrol of the time and/or temperature of the exchange reaction. By wayof illustration, the cation exchange reaction can be carried out attemperatures in the range from 350° C. to 500° C., includingtemperatures in the range from 380° C. to 450° C., and/or for timesranging from one hour to six hours. However, temperatures and timesoutside of these ranges can be used.

In_(1-x)Ga_(x)P (0<x<1) nanocrystals are examples of nanocrystals thatcan be made using cation exchange in a molten inorganic salt medium.These ternary Group III-V nanocrystals can be formed starting with InPnanoparticles dispersed in, for example, a molten NaSCN/KSCN mixture ora molten CsBr:KBr:LiBr mixture to which a gallium halide salt, such asGaI₃, is added. The degree of cation exchange can be controlled throughthe reaction temperature, with higher temperatures corresponding to alarger degree of exchange. For example, by adjusting the reactiontemperature from about 380° C. to about 425° C., nanocrystals with x inthe range from about 0.2 to about 0.85 can be obtained. The ability tosynthesize colloidal In_(1-x)Ga_(x)P nanocrystals with good opticalproperties is advantageous because In_(1-x)Ga_(x)P nanocrystals arebetter suited for display applications than are InP particles, asillustrated in the Example.

In_(1-x)Ga_(x)As (0<x<1) nanocrystals also can be made using cationexchange in a molten inorganic salt medium. These ternary Group III-Vnanocrystals can be formed starting with InAs nanoparticles dispersedin, for example, a molten NaSCN/KSCN mixture or a molten CsBr:KBr:LiBrmixture to which a gallium halide salt, such as GaI₃, is added. Thedegree of cation exchange can be controlled through the reactiontemperature, with higher temperatures corresponding to a larger degreeof exchange. For example, by adjusting the reaction temperature fromabout 400° C. to about 500° C., In_(1-x)Ga_(x)As nanocrystals with x inthe range from about 0.05 to about 0.75 can be obtained. Notably, usingthe present methods, colloidal In_(1-x)Ga_(x)As nanocrystals with astrong band edge emission in the near infrared range of 750 nm-1000 nmcan be fabricated.

The colloids may be characterized by their stability as evidenced by themaintenance of a homogeneous and uniform distribution of nanocrystalsthroughout the molten medium over a period of time. Colloidal stabilitymay be measured visually (e.g., photographs), by using TEM images, orSAXS data. At least some embodiments of the colloids are stable under aninert atmosphere for at least a month or two or more months. For longerstorage, composites may be formed by solidifying the colloids. Suchcomposites may be reheated to the molten state, thereby reforming acolloid characterized by a homogeneous and uniform distribution ofnanocrystals throughout the molten medium.

In the ternary Group III-V nanocrystals the cations that are involved inthe cation exchange are alloyed; that is, they coexist in the samecrystal domain, rather than being segregated in two or more differentdomains that have different material compositions. Thus, a ternary GroupIII-V nanocrystal is distinguishable from a nanocrystal heterostructure,such as a core-shell nanocrystal, in which the two cations that undergothe cation exchange end up segregated in different domains. However, theGroup III-V nanocrystals can be incorporated into core-shell structuresby growing a shell around the Group III-V nanocrystals. Thus, in someembodiments of the methods, the colloidal Group III-V nanocrystals areused as media for forming core-shell nanocrystals. In such embodiments,the chemical transformation involves forming a shell over the inorganicGroup III-V nanocrystals of the colloid. Growth of a thin wide band gapsemiconductor shell over nanocrystal cores is a strategy used to improvethe optical properties of semiconductor QDs. However, shell growth canbe challenging. Efficient shell growth methods typically require highlyair-free conditions and high temperature so as to avoid incorporatingstructural defects at the core-shell interface, which can negativelyaffect emission properties. Shell growth can be particularly challengingin the case of Group III-V semiconductors which are prone to theformation of oxides.

One embodiment of a method for forming core-shell nanocrystals comprisesadding a shell precursor to the colloidal Group III-V nanocrystals at atemperature and time sufficient to form a shell over the nanocrystals ofthe colloid. The shell may be composed of a semiconductor, e.g., adifferent group III-V semiconductor, or a group II-VI semiconductor. Forexample, a ZnS shell can be formed on an In_(1-x)Ga_(x)P core or a CdSshell can be formed on a In_(1-x)Ga_(x)As core, as illustrated in theExample.

The Group III-V nanocrystals can be removed from the molten salt mediumby washing with an organic solvent and/or carrying out a phase transferinto an organic solvent, followed by separating the nanocrystals fromthe solvent phase.

The Group III-V nanocrystals, including core-shell nanocrystals having aGroup III-V core, can be used in a variety of optoelectronic devices.For example, the nanocrystals can be used as photodetectors, wherebyincident radiation of a first wavelength or range of wavelengths isabsorbed by the ternary Group III-V nanocrystals, inducing them to emitPL of a second wavelength or range of wavelengths. The absorption and PLspectra of the nanocrystals will depend on their chemical composition.However, by way of illustration, In_(1-x)Ga_(x)P (0<x<1) nanocrystalscan absorb blue light and emit green and/or red PL, with a PL peakhaving a full width half maximum of 60 nm or lower, including 50 nm orlower.

EXAMPLE

This example describes a molten salt-based approach for the synthesis ofternary InGaP and InGaAs NCs via cation exchange reactions performed onpre-synthesized InP and InAs NCs (FIG. 1A). The resultant cationexchanged particles show absorption and emission features that are blueshifted in comparison to the starting materials. Bright luminescencefrom core-shell nanocrystals of InGaP/ZnS is also demonstrated, and itsoptical properties are compared with those of InP nanoparticles.

Experimental Section; Results and Discussions; Structural Properties

Colloidal InP and InAs QDs can be dispersed in a variety of inorganicsalt eutectics (Table 1) such as NaSCN/KSCN (26.3:73.7 mol %, m.p. 137°C.) or CsBr:LiBr:KBr (25:56.1:18.9 mol %, m.p. 236° C.). The nativeorganic ligands were first removed from the QD surface by either usingHBF₄ as the stripping agent or by decorating the QDs surface with shortinorganic S²⁻ ligands. Dried powders of organic ligand-free QDs werestirred in the molten salt at temperatures slightly above their meltingpoint for prolonged periods to obtain stable dispersions. Thedispersions of InP and InAs QDs in molten CsBr:LiBr:KBr were stable attemperatures well beyond 400° C. At these temperatures, traditionalorganic solvents and surface ligands either boil or decompose. Thestability of QDs in molten salts can be attributed to the ability of thesalt anions and cations to form strongly ordered templates around thenanocrystal surface (FIG. 1A). Both halide and SCN⁻ ions bind to thesurface of III-V nanocrystals, which is essential to induce enhancedordering of the ions.

TABLE 1 Molten Salt and QD combinations tested in the Example EutecticComposition Melting QDs (% mol) Temperature dispersed CsBr:LiBr:KBr =236° C. InP, InAs 25:56.1:18.9 KSCN:NaSCN = 140° C. InP, InAs 73.7:26.3ZnCl2:NaCl:KCl = 203° C. InP 60:20:20

To perform cation exchange reactions on III-V QDs, the CsBr:LiBr:KBreutectic was chosen due to its high temperature stability, low vaporpressure, high solubility of Group III halides, and inertness to InP andInAs QDs. QDs capped with sulfide ligands showed better stability inthis eutectic mixture and also did not show Ostwald ripening in thesesalts. A desired amount of GaI₃ salt (m.p.=212° C.) was added to theQD/molten salt solution and it was stirred for 2 hours (h) at ˜250° C.to allow homogenization. The mixture was then transferred to a furnaceand heated to 380° C.-450° C. for an hour. According to the HSABprinciple, the softer In³⁺ should have a higher preference than Ga³⁺ forthe soft iodide ion favoring the exchange. The reaction mixture wascooled down to room temperature and the salt was removed by repeatedwashing using formamide (FA) in inert atmosphere. The centrifugedproduct was then dispersed in FA using sulfide ligands and transferredto toluene phase using didodecyldimethylammonium bromide (DDAB) as thephase transfer agent to obtain a colloidal solution which was stable formonths.

Gallium pnictides are thermodynamically more stable than theircorresponding indium pnictides. For example, the standard heat offormation of GaP and InP is −103.2 kJ/mol and −70.2 kJ/mol, respectively(−87.7 kJ/mol and −60 kJ/mol for GaAs and InAs). Therefore, the cationexchange is only diffusion limited and can be accelerated by increasingthe reaction temperature. The extent of Ga incorporation in the QDscould indeed be controlled by the temperature at which the exchange wasperformed. FIG. 2A shows XRD patterns of In_(1-x)Ga_(x)P nanocrystalswith varying compositions obtained from cation exchange at temperaturesranging from 380° C.-430° C. A consistent shift of all X-ray reflectionsto higher 2θ values was observed with increasing temperature, whichindicates increasing Ga incorporation into the lattice with temperature.Similar results were obtained for In_(1-x)Ga_(x)As NCs (FIG. 2B);however, the temperature range needed for Ga incorporation was 400°C.-450° C. No change in XRD patterns was observed when the particleswere annealed in the absence of GaI₃. The full width at half maximum(fwhm) of the (111) diffraction peak did not appreciably change when thecation exchange reactions were performed at temperatures below 450° C.,indicating that the QDs did not grow or etch significantly. Although thecomposition could be driven almost completely to the GaAs phase when theexchange was performed at 500° C., it was accompanied by significantnarrowing of the diffraction peaks, indicative of an increase in theparticle size. The composition of the alloy was estimated from thelattice parameters using the Vegard's law (FIGS. 2A and 2B). Inductivelycoupled plasma optical emission spectrometry (ICP-OES) analysis of theQDs was found to be in a good agreement with the compositions estimatedfrom the XRD patterns. Further insight into the nature of alloying couldbe obtained from Raman spectroscopy. FIGS. 2C and 2D show the Ramanspectra for different alloy compositions of In_(1-x)Ga_(x)P andIn_(1-x)Ga_(x)As QDs. A continuous one-mode shift in the TO and LOphonon modes of the parent InP and InAs phase could be seen forIn_(1-x)Ga_(x)P and In_(1-x)Ga_(x)As QDs with increasing Gaincorporation, indicating that the alloy QDs did not have phasesegregated domains of InP and GaP. The TO and LO phonon features for thealloys with a higher Ga component showed significant broadening,indicating the lack of a long-range order between In and Ga sub-latticesin the alloy QDs.

The morphology of the alloy In_(1-x)Ga_(x)P and In_(1-x)Ga_(x)As QDs wascharacterized using electron microscopy. FIG. 3A shows a representativeSTEM image of In_(1-x)Ga_(x)P QDs. A high-resolution STEM image showingclear lattice fringes is shown in the inset. The homogeneity ofstructural alloying is further supported by high resolution energydispersive X-ray (EDX) chemical mapping of In_(1-x)Ga_(x)P QDs (FIG.3B). The presence of both indium and gallium could be detected in eachindividual QD without any apparent phase segregation. Elemental linescans on individual QDs show a STEM-EDS map of an individualIn_(1-x)Ga_(x)As QD showing the presence of both In and Ga atoms in it.A significant change was not observed in the size of QDs upon cationexchange. FIGS. 3C and 3D show TEM images of the starting InP QDs andthe resultant In_(0.6)Ga_(0.4)P QDs obtained after cation-exchange at410° C. for comparison. SAXS analysis was employed to quantitativelystudy the change in the size and size distribution of the QDs uponcation exchange (FIG. 3E). The average size of In_(0.6)Ga_(0.4)P QDsshrunk by ˜3%, which is consistent with the change in volume expecteddue to the smaller unit cell of GaP compared to InP (a_(InP)=0.586 nmand a_(GaP)=0.545 nm). The size distribution of 11% for the initial InPdots increased to 13.5% after the cation exchange. Similar results wereobtained when cation exchange was performed on InAs QDs. TEM images ofIn_(0.5)Ga_(0.5)As QDs and the starting InAs QDs are shown in FIGS. 8Aand 8B. High resolution TEM (HRTEM) images of In_(1-x)Ga_(x)P andIn_(1-x)Ga_(x) As QDs are shown in FIGS. 9A and 9B. The distance between(111) reciprocal lattice points was estimated from the Fourier transformof the HRTEM image of an individual alloy QD, which pointed to theshrinking of unit cell dimension from a=5.9 Å to a=5.6 Å.

Next, the effect of Ga incorporation on the optical properties of InPQDs was studied. Alloyed In_(1-x)Ga_(x)P QDs are expected to showcontinuously blue shifted absorption with increasing Ga content ascompared to InP QDs of the same size, owing to the larger band gap ofGaP (E_(g,bulk)=2.3 eV(indirect)/2.77 eV(direct)) as compared to InP(E_(g,bulk)=1.34 eV). FIG. 4A shows the absorption spectra ofIn_(1-x)Ga_(x)P QDs synthesized at different temperatures. The excitonicfeatures were continuously blue shifted with increasing values of x forIn_(1-x)Ga_(x)P alloy QDs. No blue shift of absorption was observed whenInP QDs were annealed in molten salts without GaI₃. Similar blue shiftswere observed upon alloying in InP QDs of all sizes (FIG. 4B). Theextent of blue shift was found to vary for different QD sizes and wasnot linearly correlated to the % of Ga in the lattice, which can beattributed to the band-bowing effect in ternary In_(1-x)Ga_(x)P.

The absorption features for the alloy QDs are generally broader ascompared to the starting materials, which can be attributed to twofactors: (1) there may be some heterogeneity in the distribution of Gain the ensemble, and (2) a slight change in size distribution is alsoobserved after cation exchange. Both these factors can be mitigated bymechanical stirring of the reaction mixture during the high temperaturecation exchange. Further optimization and scale up of this process canresult in better size dispersions. Mild size-selective precipitation wasused to partially eliminate the effect of ensemble heterogeneity on theoptical properties of the alloy QDs. Size-selective precipitationallowed for the separation of the particles into smaller batches withtighter size distributions and narrower excitonic features. Theabsorption features for a size-selected fraction of In_(0.6)Ga_(0.4)PQDs and InP QDs were slightly broader for the alloy QDs as compared tothe binary phase, which can be attributed to either heterogeneity in Gaincorporation or intrinsic differences in the exciton fine structure.

As-synthesized In_(1-x)Ga_(x)P QDs also showed band-edge emission, whichwas blue shifted as compared to the starting InP NCs. FIG. 4C showsrepresentative emission spectra of alloy In_(1-x)Ga_(x)P QDs with theircorresponding absorption spectra. The full width at half maximum for thePL band was less than 50 nm for all size ranges. The Stokes shift forIn_(1-x)Ga_(x)P QDs is comparable to that obtained for InP particlesemitting at similar wavelengths. The molar extinction coefficient ofIn_(0.6)Ga_(0.4)P alloy QDs was also estimated, and it was compared toInP QDs with a similar emission spectrum as that of the alloy QDs (FIG.4D). The extinction coefficient per particle for the alloy QDs was foundto be significantly higher than that of InP QDs in the blue spectralrange. This is expected since the extinction coefficients of QDs scalelinearly with the number of unit cells, which is higher for InGaP thanInP for the same QD size. The absorption cross-section at 450 nm for thealloy QDs with emission maxima centered at 576 nm was found to be 1.5times that of InP QDs emitting at the same wavelength. This is of greatsignificance for display applications where blue light is used to excitethe green and red emitting QD phosphors.

The emission quantum yields of In_(1-x)Ga_(x)P QDs could besignificantly enhanced upon shell growth. Core-only alloy QDs showedemission quantum yields in the range of 1-5%, which increased to 46%upon ZnS shell growth (FIG. 5A). Shell growth could be tracked bymonitoring absorption below 350 nm. A type-I band alignment is expectedfor these core-shell QDs, which is evidenced by the lack of substantialred-shift upon shell growth. In_(1-x)Ga_(x)P/ZnS core-shell QDs emittingin the range of ˜495 nm to ˜640 nm could be prepared with quantum yieldsin the range of 30-40% routinely observed across the size range (FIG.5B). The incorporation of 50% Ga in InP QDs should reduce the latticemismatch with the ZnS shell from 7.5% to 4%, which can substantiallyalleviate the interfacial strain between the core and shell. PLexcitation spectra of In_(0.5)Ga_(0.5)P/ZnS and InP/ZnS core-shellnanoparticles of similar sizes were measured (FIG. 5C). Significantlynarrower PLE spectra were observed for In_(0.5)Ga_(0.5)P/ZnS QDs incomparison to InP/ZnS QDs of similar sizes (FIG. 5C). The broadening ofPLE spectra for InP/ZnS core shells can be attributed to the latticestrain in these systems. A graded composition of the In_(1-x)Ga_(x)Pcore can also explain this observation, which protects significantincorporation of Zn into the QD core, thereby reducing disorder relatedStokes shift. A series of PLE spectra collected at different positionsof the emission band showed a significant distribution of transitionenergies in the ensemble (FIG. 10).

For the technological implementation of QDs in LEDs, display panels, andsolar concentrators, the retention of luminescence efficiencies at hightemperatures (up to 150° C.) is an important requirement. The loss inluminescence efficiency at high temperatures is typically attributed tothermally activated trapping of individual carriers. Althoughluminescence retention at high temperatures is significantly enhancedupon shell growth, factors like core size, synthesis temperature, andinterfacial strain are known to play important roles in determining thethermal stability of PL. The effect of Ga incorporation on the thermalstability of PL in InP/ZnS QDs was examined. InP/ZnS QDs and theIn_(0.6)Ga_(0.4)P/ZnS alloy QDs with similar core sizes were immobilizedin a crosslinked poly(lauryl methacrylate) matrix, and their PL wasmeasured at elevated temperatures upon excitation with a 473 nm laser(FIGS. 6A and 6B). Whereas the PL of InP/ZnS QDs decreased drasticallybeyond 100° C. and reduced to ˜30% at 150° C., the PL ofIn_(0.6)Ga_(0.4)P/ZnS NCs only decreased to ˜60% upon heating to 150° C.(FIG. 6C). Similar results were obtained when temperature dependent PLstudies were performed in solution (FIGS. 6D, 11A, and 11B). Thissignificantly better performance of the alloy QDs can be attributed tothe reduced lattice mismatch and the resultant low strain at thecore-shell interface.

The optical properties of In_(1-x)Ga_(x)As alloy QDs were qualitativelysimilar to In_(1-x)Ga_(x)P QDs. A continuous blue shift of theabsorption edge was observed with increasing Ga content in the alloy QDs(FIG. 7A). The excitonic features of the alloys were broader as comparedto the initial InAs QDs. Strong band-edge luminescence could be seenupon growing a shell of CdS on In_(1-x)Ga_(x)As. Shell growth wasaccompanied by a slight red shift of the excitonic band, likely due tothe leakage of the electron wavefunction into the shell. A highestquantum yield of 9.8% was obtained for In_(0.5)Ga_(0.5)As/CdS QDs withemission centered at ˜860 nm (FIG. 7B). Emission wavelength could betuned in the biological tissue transparent window of ˜750 nm-950 nm bysimply varying the alloy composition (FIG. 12). InAs QDs can be used asnear infrared (NIR) emitting probes for in-vivo biological imaging. Apotential disadvantage of InAs QDs for these applications is that InAsQDs have to be less than ˜3 nm in size to emit in the window of ˜700-950nm, which makes them rather unstable. Alloying of Ga in InAs QDs affordslarger-sized QDs emitting in this region. The size of In_(0.5)Ga_(0.5)AsQDs with an excitonic feature around ˜800 nm was estimated to be ˜4 nm.3D PL contour maps on luminescent In_(0.5)Ga_(0.5)As/CdS QDs were alsomeasured to estimate the effect of size homogeneity on their absorptionand emission properties. The diagonal elongation of spectra features inthese PL intensity maps shows that the sample consists of an ensemble ofQDs with a range of transition energies. Slicing of these maps gave aseries of PL excitation spectra which showed narrow excitonictransitions. Similarly, PL line narrowing experiments allowed for theselective excitation of only a fraction of QDs in the ensemble, whichshowed narrow PL spectra. Both these experiments demonstrate that thebroad emission linewidths in the alloy QDs are not inherent to the alloyand can be improved by further optimization of the synthesis conditions.

Experimental Section

All manipulations with molten salts were performed in a nitrogen filledglove box. The synthesis of InP and InAs QDs was performed based onreported protocols. (Ramasamy, P. et al., Chemistry of Materials 2017,29 (16), 6893-6899; Srivastava, V. et al., Chemistry of Materials 2018;and Battaglia, D. et al., Nano Letters 2002, 2 (9), 1027-1030.) Detailsof these syntheses are given in the Additional Experimental Details,below. Details of characterization techniques are also provided below.

Ligand Exchange on InP and InAs QD.

The purified InP and InAs QDs (˜0.3 mmol QDs) were transferred to thepolar formamide (FA) phase using (NH₄)₂S as inorganic capping ligands.200 μL volume of 40-48% aq. (NH₄)₂S solution in 10 mL FA and QDssuspended in toluene were stirred together for 20 min to completelytransfer the particles to the polar FA phase. The organic phase wasremoved and fresh toluene was added and the biphasic mixture was stirredfor another 15 mins. This process was repeated thrice to completelyremove all the organic ligands. The particles were colloidally stable inFA. The particles could be precipitated using excess CH₃CN and dried aspowders. These powders were further used for dispersion in the moltensalts.

Alternatively, the particles could be transferred from the FA phase totoluene using DDAB as the phase transfer agent. This ligand decomposescleanly into gaseous products via Hoffman elimination, leaving noorganics behind. The toluene phase containing QDs was transferred to acentrifuge tube and precipitated with ethanol to get rid of excessligands and re-dispersed in 2-3 mL of toluene. This solution was usedfor dispersion in molten salts. Absorption spectra before and after theligand exchange were measured in toluene.

Bare InP QDs were prepared by stripping with HBF₄ using previouslyreported protocols. (Nag, A. et al., Journal of the American ChemicalSociety 2011, 133 (27), 10612-10620.) Bare InP QDs were used as powdersfor dispersion in the molten salts.

Dispersion of InP and InAs QDs in Molten Salt Matrix and Cation Exchangeinto In_(x)Ga_(1-x)P and In_(x)Ga_(1-x)As QDs.

A eutectic mixture of CsBr:LiBr:KBr (25:56.1:18.9 mol %, (melting point236° C.)) was taken in a vial and heated to 250° C. under inertatmosphere until a complete liquid phase was formed. The molten salt wascooled to room temperature (r.t.) and grinded into a fine powder. ˜0.3mmol InP/InAs QD powders capped with S²⁻ ligands were then added aspowder or as a toluene solution (see section above) to the finallygrinded eutectic mixture and heated to 275° C. under stirring for a fewhours until a stable solution was obtained. Similar protocols were usedfor the dispersion of QDs in other molten salts. For cation exchange,˜4-8 molar equivalents of GaI₃ (0.5 g-1 g GaI₃) was added to theQD/molten salt dispersion as a source of Ga³⁺ cations. GaCl₃ or GaBr₃could also be added as a source of Ga³⁺; however, best results wereobtained with GaI₃ due to its higher boiling point. The mixture was thenfurther heated at 300° C. for 1 h to completely homogenize the QDs anddopant salt. The mixture was cooled to r.t. and then transferred to afurnace, where it was further heated at a desired temperature (380°C.-500° C.) for 1 h in N₂ atmosphere. The mixture was cooled to r.t. andthe salt matrix was dissolved using excess FA. The cation exchanged QDswere centrifuged. The QDs were washed twice with FA to completely removethe salt matrix. Finally, the QDs were re-dispersed in FA using (NH₄)₂S(˜100 uL in 10 mL FA) and transferred to toluene using DDA⁺ as thecounter-ion and used for further characterization. This surface cappingprocedure is similar to the one described above. Size selection of thecrude solution in toluene into a desired number of fractions wasperformed by sequential precipitation with an appropriate amount ofethanol.

ZnS Shell Growth on In_(1-x)Ga_(x)P QDs

In a 50 mL 3-neck flask, 0.4 mmol of Zn (OAc)₂ and 1 mmol of Oleic acidwere mixed in 6 mL ODE. The solution was heated to 120° C. for an hourand cooled to r.t. A toluene solution of the cation exchanged andsize-selected In_(1-x)Ga_(x)P/S²⁻/DDA⁺ QDs was injected into thissolution. Toluene was first evaporated under vacuum at 60° C., and thesolution was then heated to 280° C. under N₂. 0.3 mmol S in 3 mL TOP wasthen injected into this solution at the rate of 1 mL/h using a syringepump. The reaction temperature was ramped to 300° C., 30 min after thesyringe pump injection began. The reaction mixture was cooled to r.t.after the reaction was completed, and the core-shell QDs were washedusing toluene/ethanol as the solvent and non-solvent.

CdS Shell Growth on In_(1-x)Ga_(x)As QDs

The protocol was similar to the protocol for ZnS shells. Pre-synthesizedCd-oleate was used in this case as the Cd precursor. The reactiontemperature was held at 240° C.

Preparation of QD/Polymer Composites

The polymer composites containing InP/ZnS and In_(1-x)Ga_(x)P/ZnS QDswere prepared according to a reported protocol. (Zhao, Y. et al., ACSNano 2012, 6 (10), 9058-9067.) The monomer lauryl methacrylate (80 wt %)and cross-linker ethylene glycol dimethyacrylate (20 wt %) were mixedtogether. A toluene solution of QDs was precipitated using ethanol andre-dispersed in this mixture in a low concentration. For polymerization,0.3 wt % of azobisisobutyronitrile (AIBN) was added to this mixture, andthe combination was heated to 70° C. overnight.

Additional Experimental Details

Chemicals

Indium(III) chloride (InCl₃, anhydrous, 99.99%, Puratrem), In(III)acetate (In(OAc)₃, 99.99%, Aldrich), tris(trimethylsilyl) phosphine((TMS)₃P, 98%, Strem Chemicals), alane N,N-dimethylethylamine complex(DMEA-Al, 0.5 M solution in toluene, Aldrich), oleylamine (70%,Aldrich), octadecene (90%, Aldrich), oleic acid (90%, Aldrich),tris(dimethylamino) arsine (As(NMe₂)₃, 99%, Strem), ammonium sulfide(40-48% in water, Aldrich), formamide (99.5%, Aldrich),didodecyldimethylammonium bromide (DDAB, 98%, Aldrich), potassiumbromide (ultra-dry, 99.9%, Alfa Aesar), cesium bromide (ultra-dry,99.9%, Alfa Aesar), lithium bromide (ultra-dry, 99.9%, Alfa Aesar),gallium iodide (ultra-dry, 99.999%, Alfa Aesar), toluene (anhydrous,99.8%, Aldrich), ethyl alcohol (anhydrous, ≥99.5%, Aldrich), zincacetate (99.99%, Aldrich), sulfur (99.998%, Aldrich), trioctylyphosphine(97%, Strem Chemicals), lauryl methacrylate (96%, Aldrich), ethyleneglycol dimethacrylate (98%, Aldrich). Oleylamine, octadecene andformamide were dried under vacuum before use.

Synthesis of Small InP QDs

Small InP QDs (λ_(max)=500-540 nm) were synthesized using a slightmodification of a reported protocol. (Ramasamy et al., 2017) Indiumacetate (0.45 mmol) and oleic acid (1.4 mmol) were mixed with 10 mL ofODE in a 50 mL three-neck flask and fixed to a Schlenk line with areflux condenser. The mixture was heated to 120° C. under vacuum for 1h. Then, the flask was refilled with N₂ and cooled to r.t. Then, asolution containing 0.3 mmol (0.25 for particles with λmax=540 nm) of(TMS)₃P and 1 mL of TOP was quickly injected into the flask. Followingthe injection, the mixture was heated to 305° C. (15° C./min) and keptat that temperature for 2 min before cooling to r.t. The QDs wereprecipitated with 50 mL of ethanol and collected by centrifugation. TheQDs were washed three times by dispersion in hexane, followed byprecipitation by addition of ethanol, and stored in hexane in a vial ina N₂ filled glovebox.

Synthesis of Large InP QDs

Large InP QDs (λ_(max)>600 nm) were synthesized using a slightmodification of reported protocol. Indium acetate (0.45 mmol) andmyristic acid (4-6 equivalent, depending on size) were mixed with 10 mLof ODE in a 50 mL three-neck flask and fixed to a Schlenk line with areflux condenser. The mixture was heated to 120° C. under vacuum for 1h. Then, the flask was refilled with N₂ and cooled to r.t. Then, asolution containing 0.3 mmol (0.25 for particles with λmax =540 nm) of(TMS)₃P and 1 mL of TOP was quickly injected into the flask. Followingthe injection, the mixture was heated to 305° C. (15° C./min) and keptat that temperature for 2 min before cooling to r.t. The QDs wereprecipitated with 50 mL of ethanol and collected by centrifugation. TheQDs were washed three times by dispersion in hexane, followed byprecipitation by addition of ethanol, and stored in hexane in a vial ina N₂ filled glovebox.

Stock Solution of as Precursor for InAs QDs

0.4 mmol As(NMe₂)₃ was dissolved in 1 mL dry oleylamine and kept at 40°C. for 2 mins until bubbles stopped evolving. This bubble formationindicates transamination and the evolution of methylamine.

Synthesis of InAs QDs

InAs QDs were synthesized according to a reported protocol. (Srivastavaet al., 2018) In a typical synthesis, 0.4 mmol InCl₃ and 6 mL oleylaminewere loaded in a 100 mL 3-neck flask and dried at 120° C. under vacuumfor 1 h. The reaction mixture was then brought to a desired temperature(150° C.-220° C.) depending on the target QD size. The As stock solutionwas then quickly injected into the flask followed by the injection of2.4 mL of 0.5 M DMEA-Al in toluene. The temperature was then furtherincreased to 240° C.-290° C. depending on the target QD sizes. Thereaction was cooled to r.t. and transferred into a N₂ filled glovebox.The reaction mixture was diluted with 5 mL toluene, and 15 mL ethylalcohol was added to precipitate the QDs. The washing cycle was repeatedtwice, and the precipitated particles were redispered in toluene.

Characterization Techniques

TEM: The images were obtained using 300 KV FEI Tecnai F30 microscope.Samples for TEM were prepared by depositing one droplet of dilutednanocrystal solution in toluene onto a lacey carbon grid from Ted Pella.

Optical absorption measurements: QDs dispersed in toluene were used forabsorption measurements. The absorption spectra of the solutions werecollected using a Cary 5000 UV-Vis-NIR spectrophotometer. PL and PLEspectra were collected Horiba Fluorolog 3 equipped with Si CCD detectorsensitive up to 1050 nm.

Wide-angle powder XRD: The diffraction patterns were obtained using aBruker D8 diffractometer with Cu Kα X-ray source operating at 40 KV and40 mA and Vantec 2000 area detector.

ICP-OES: ICP-OES analysis was carried out using an Agilent 700 Seriesspectrometer. Samples were digested by a mixture of deionizedultrafiltered water and nitric acid (HNO₃, ≥69.0%, TraceSELECT, fortrace analysis, from Sigma Aldrich) or aqua regia (HCl used for aquaregia was purchased from Sigma Aldrich, ≥37%, TraceSELECT, for traceanalysis, fuming) in a plastic container.

Raman Spectroscopy Measurements: Raman spectra were collected on filmsdeposited on glass using a Horiba LabRamHR Evolution confocal Ramanmicroscope. The samples were excited using a 532 nm laser. Hightemperature PL measurements on QD/Polymer composites were also carriedout on this instrument. The composites were excited using a 473 nm laseroperating at 0.1% of its power using a 50× long path objective. The PLwas detected using the Synapse detector.

SAXS Measurements: SAXS measurements on the colloidal NPs were collectedon a SAXSLab Ganesha instrument with Cu Kα radiation. The SAXS curveswere analyzed by fitting to a quantitative model in Igor Pro using theIrena package. (Ilavsky, J. et al., J. Appl. Cryst. 2009, 42, 347-353.)All scattering curves were firstly fitted with the model independentmaximum entropy approach to make sure that the size distributions weresymmetric Gaussian. Then the size distributions were extracted using theModelling II module in the Irena package. Based on TEM data, theparticles' form factor was assumed to be spherical with the aspect ratioof 1.

Estimation of Quantum Yields

Quantum yields were calculated with respect to Rhodamine 6G (forIn_(1-x)Ga_(x)P) or IR-125 dye (for In_(1-x)Ga_(x)As) as the referencedyes with solution optical density at the excitation wavelength between0.05 and 0.1. PL spectra were taken and the integrated areas were usedin the following calculation of quantum yield:

$\varphi = {\varphi_{ref}\;\frac{I_{x}}{I_{ref}}\frac{A_{ref}}{A_{x}}\frac{\eta_{st}^{2}}{\eta_{ref}^{2}}}$where x is the sample and ref is Rhodamine 6G/IR-125. The quantum yieldof Rhodamine 6G was taken as 0.95 and IR-125 as 0.134. I represents theintegrated emission intensity, A represents the optical density in theabsorption spectra, and n representing the refractive indices of thesolvents. It is important to note that the samples were excited at thewavelength where optical density is equal.Estimation of Molar Extinction Coefficients

In order to estimate the values of the molar extinction coefficient ofInP and In_(1-x)Ga_(x)P QDs, thoroughly washed samples werecharacterized by UV-Vis spectroscopy and the particles were subsequentlydissolved in aqueous HNO₃ digesting solution. The total metal (In+Ga)concentration of each sample was determined by ICP-OES. Once the metalconcentrations were determined, the particle concentrations werecalculated by using bulk lattice parameters and the average particlesize, which were determined by TEM and SAXS measurements.

The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure and unless otherwise specified, “a” or “an” means “oneor more.”

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A method for forming ternary Group III-Vnanocrystals, the method comprising: dispersing binary Group III-Vnanocrystals in a molten inorganic salt; adding an ion-exchange additivecomprising a Group III element or a Group V element to the molteninorganic salt; heating the molten inorganic salt for a time and at atemperature at which the Group III element or the Group V element of thebinary Group III-V nanocrystals and the Group III element or the Group Velement of the ion-exchange additive undergo cation exchange to form theternary Group III-V nanocrystals.
 2. The method of claim 1, wherein theion-exchange additive is a molten inorganic salt of a Group III elementor an inorganic salt of a Group V element.
 3. The method of claim 1,wherein the ion-exchange additive is a gaseous compound of a Group IIIelement or a gaseous compound of a Group V element.
 4. The method ofclaim 2, wherein the ion-exchange additive is the molten inorganic saltof the Group III element, and further wherein said molten inorganic saltof the Group III element is a halide salt of the Group III element. 5.The method of claim 2, wherein the binary Group III-V nanocrystals aresurface functionalized with inorganic ligands that enhance theirsolubility in the molten inorganic salt.
 6. The method of claim 5,wherein the inorganic ligands are sulfide ligands.
 7. The method ofclaim 2, wherein the binary Group III-V nanocrystals are InPnanocrystals, and the ternary Group III-V nanocrystals areIn_(1-x)Ga_(x)P nanocrystals, where 0<x<1.
 8. The method of claim 7,wherein the ion-exchange additive is the molten inorganic salt of theGroup III element, and further wherein said molten inorganic salt of theGroup III element is GaI₃.
 9. The method of claim 7, further comprisinggrowing a shell of semiconductor material on the ternary Group III-Vnanocrystals.
 10. The method of claim 2, wherein the binary Group III-Vnanocrystals are InAs nanocrystals, and the ternary Group III-Vnanocrystals are In_(1-x)Ga_(x)As nanocrystals, where 0<x<1.
 11. Themethod of claim 10, wherein the ion-exchange additive is the molteninorganic salt of the Group III element, and further wherein said molteninorganic salt of the Group III element is GaI₃.
 12. The method of claim10, further comprising growing a shell of semiconductor material on theternary Group III-V nanocrystals.
 13. The method of claim 2, wherein themolten inorganic salt in which the binary Group III-V nanocrystals aredispersed is a mixture of two or more inorganic salts.
 14. The method ofclaim 13, wherein the mixture of two or more inorganic salts comprisesNaSCN and KSCN.
 15. The method of claim 13, wherein the mixture of twoor more inorganic salts comprises CsBr, KBr, and LiBr.
 16. The method ofclaim 2, further comprising growing a shell of semiconductor material onthe ternary Group III-V nanocrystals.
 17. The method of claim 2, whereinthe temperature is in the range from 350° C. to 500° C.